[0001] This invention relates to a sensor for sensing the displacement of a body and in
particular to a sensor which is suitable for use in hostile environments where direct
contact with the body is undesirable.
[0002] It is known for displacement to be sensed using a follower device such as disclosed
in US-3065635 in which a magnetic follower is isolated from the body but moves in
unison with the body by virtue of magnetic coupling. The displacement of the follower
device is then taken as an indication of the body's displacement.
[0003] Such sensors have particular application to measurement of liquid levels where the
body constitutes a float.
[0004] It is also known from US-4637254 to provide a position sensor in which a deformable
strip adopts an S-bend configuration at a localised position along its length and
the position at which the S-bend is formed is determined by the position of a body
whose displacement is to be sensed. The position at which the S-bend is formed is
then detected as an indication of body position. A disadvantage of this system is
that the body is biassed into contact with the strip in order to deform the strip
and a flexible seal arrangement is required to protect the sensor from the environment
in which the body is movable.
[0005] According to the present invention there is disclosed a sensor for sensing the displacement
of a body comprising a deformable strip, a rigid enclosure defining a chamber within
which the strip is received and isolating the strip from contact with the body, constraining
means operable to maintain the strip under longitudinal and transverse constraint
such that first and second end portions of the strip extend substantially parallel
to a longitudinal direction in which the body is movable relative to the sensor and
locally deforming the strip in a transverse direction orthogonal to the first and
second portions such that a deformed portion in the form of an S-bend integrally and
unitarily connects the first and second end portions, and first magnetic means operable
between the strip and the body to apply a magnetic force non-uniformly to the strip
such that the location at which the strip is deformed to constitute the deformed portion
is longitudinally variable in registration with a reference portion of the body whereby
the longitudinal displacement of the deformed portion is representative of the displacement
of the body.
[0006] An advantage of such an arrangement is that the rigid enclosure avoids the need for
any flexible seal arrangement and no contact is made between the body and the strip
thereby reducing frictional drag effects on body movement.
[0007] Preferably the strip is comprised of ferromagnetic material and the body comprises
a magnetic element arranged in closer proximity to the first end portion than to the
second end portion thereby constituting the first magnetic means.
[0008] Conveniently the length of the length of the chamber is less than the undeformed
length of the strip whereby end walls of the enclosure constitute longitudinally constraining
elements of the constraining means.
[0009] Preferably the enclosure comprises longitudinally extending first and second walls
contacted by the respective first and second end portions of the strip and wherein
the first magnetic means is operable to bias the first end portion into contact with
the first wall.
[0010] Preferably the sensor further comprises biassing means operable to bias the second
end portion into co-planar contact with the second wall.
[0011] The biassing means may comprise a second magnetic means operable to apply substantially
uniformly to the strip a magnetic force in opposition to and of smaller magnitude
than the magnetic force applied by the first magnetic means.
[0012] Alternatively the biassing means may comprise electrostatic means operable to apply
an electrostatic force in opposition to and of lesser magnitude than the magnetic
force applied by the first magnetic means.
[0013] Alternatively the biassing means may comprise a clamp operable on the second linear
portion to apply a transverse deformation such that the second linear portion exhibits
an arcuate profile when viewed in longitudinal projection.
[0014] Conveniently the magnetic element extends longitudinally in a direction parallel
to the direction of motion and wherein the magnetic element is transversely magnetised
along its length.
[0015] Advantageously the enclosure comprises a window through which the strip is visible.
[0016] The chamber may be filled with a liquid. Where an opaque liquid is used in combination
with a transparent window against which the second end portion of the strip is biassed,
the strip will be visible to the extent that it is in contact with the transparent
wall and thereby provides a visible indicator of the position of the deformed portion.
A viscous liquid within the chamber may also be utilised to provide damping.
[0017] The sensor may further comprise sensing means operable to sense the position of the
deformed portion and to produce an electrical signal representative of the longitudinal
displacement of the deformed portion.
[0018] Preferably the sensing means comprises at least one electrode associated with the
enclosure and wherein the sensing means is operable to produce an electronic signal
responsive to the capacitance between the electrode and the strip.
[0019] Alternatively the sensing means may comprise two or more inductive elements associated
with the enclosure and wherein the sensing means is operable to produce an electronic
signal responsive to inductive coupling between the elements.
[0020] Alternatively the sensing means may comprise a transmitter of electromagnetic radiation
operable to transmit longitudinally through the chamber, a detector operable to detect
reflective radiation, ranging means operable to generate an output signal representative
of the range at which the radiation is reflected and reflecting means associated with
the deformed portion whereby the output signal is representative of the displacement
of the deformed portion.
[0021] In such an arrangement the constraining means may conveniently be arranged to induce
an S-bend at the deformed portion in which the strip is deformed through substantially
90° relative to its longitudinal extent.
[0022] The sensing means may alternatively comprise a longitudinally extending fluorescent
light guide, the sensor further comprising a light source operable to illuminate the
deformed portion and an optical detector operable to produce a signal representative
of the longitudinal location of the light guide at which the fluorescence is detected.
[0023] In such an arrangement the constraining means is preferably operable to deform the
strip such that the strip is deformed through substantially 45° at the S-bend whereby
a longitudinally directed light beam is transversely deflected through substantially
90° at the deformed portion.
[0024] The sensing means may alternatively comprise a scale extending longitudinally of
the enclosure and optical reading means operable to read the portion of scale in registration
with the deformed portion.
[0025] In such an arrangement the scale may comprise markings applied to the strip and wherein
the deformed portion comprises an S-bend in which the strip is bent through substantially
90° relative to its longitudinal extent.
[0026] The scale may alternatively comprise markings applied to a wall of the enclosure
and wherein the deformed portion comprises an S-bend in which the strip is bent through
substantially 45° relative to its longitudinal extent.
[0027] In such an arrangement the optical detector comprises one or more lenses or mirrors
mounted on a sledge coupled to the strip so as to be localised to and movable with
the deformed portion.
[0028] Preferred embodiments of the present invention will now be described by way of example
only and with reference to the accompanying drawings of which:-
Figure 1 is a schematic perspective view of a sensor in accordance with the present
invention in which the strip is magnetically biassed;
Figure 2 is a schematic perspective view of an alternative embodiment in which the
strip is biassed by means of being clamped in curvilinear profile;
Figure 3 is a schematic perspective view of inductive elements of a means for sensing
the longitudinal displacement of the deformed portion of the strip;
Figure 4 is a perspective view of detail of a further alternative embodiment in which
a scale on the strip is optically read;
Figure 5 is a schematic view of a further alternative embodiment in which the body
is a float which is movable to indicate a liquid level;
Figure 6 is a schematic sectioned elevation of a further alternative embodiment in
which a sensing means comprises a light transmitter and receiver and wherein the strip
is deformed through 90°; and
Figure 7 is a schematic sectioned elevation of a further alternative embodiment in
which a sensing means is a longitudinally extending fluorescent light guide and the
strip is deformed through 45°.
[0029] In describing the following embodiments, the same reference numerals will be used
where appropriate for corresponding elements of different embodiments.
[0030] In Figure 1 a sensor 1 comprises a deformable strip 2 of resilient ferromagnetic
sheet material which extends longitudinally within a chamber 3 defined by an enclosure
4. The enclosure 4 is generally rectangular having opposite and parallel first and
second walls 5 and 6 respectively extending horizontally with the second wall uppermost
and side walls (not shown) which are removed in the drawing for clarity. The chamber
is bounded by end walls 7 and 8 against which the ends of the strip 2 abut, the end
walls 7 and 8 being longitudinally spaced apart by a distance which is less than that
required to allow the strip to extend linearly to its full extent so that, in order
to accommodate the strip within the chamber 3, the strip must be deformed.
[0031] The shape adopted by the strip 2 is such that a first portion 9 extends linearly
in contact with the first wall 5 and extends into abutment with end wall 8. A second
portion 10 extends linearly in contact with the second wall 6 so as to extend parallel
to and spaced above the first portion 9 and extends longitudinally into abutment with
end wall 7. A deformed portion 11 connects the first and second portions 9 and 10,
the first, second and deformed portions thereby being formed integrally and unitarily
and merging smoothly from one to the other.
[0032] The enclosure 4 provides longitudinal constraint upon the strip 2 by virtue of the
abutting end faces 7 and 8 which restrict the longitudinal extent of the strip and
furthermore the enclosure provides transverse constraint in a direction orthogonal
to the walls 5 and 6 and to the plane of the strip by virtue of the first and second
walls 5 and 6 being spaced apart by a fixed distance determined by the end walls 7
and 8. The position at which the strip 2 is deformed to constitute the localised deformed
portion 11 is not uniquely determined by this constraint and by applying an external
force to the strip it is possible to shift the location of the deformed portion longitudinally
in a continuously variable manner between the end walls 7 and 8. In referring to the
first, second and deformed portions 9, 10 and 11 therefore it is to be understood
that these references apply to the instantaneous shape of the strip 2 since for example
an element of the strip at its mid-point could at various times lie within the first
portion, the second portion or the deformed portion depending on the location of the
S-bend induced by constraining the strip.
[0033] A magnetic element 12 constituting part of a body whose position is to be sensed
has a major surface 13 exending parallel to and in proximity with the first wall 5
of the enclosure 4. The magnetic element 12 is longitudinally displaceable by movement
of the body in a direction parallel to the longitudinal extent of the enclosure 4
and the strip 2 and the enclosure is positioned such that the available range of movement
of a leading edge 14 of the element 12 lies between limits which are between the end
walls 6 and 7 and which are inwardly spaced from the end walls by at least the longitudinal
extent of the deformed portion.
[0034] The magnetic element 12 is permanently magnetised in a transverse direction parallel
to the end walls 6 and 7 such that the strip 2 is magnetically attracted towards the
element by a magnetic force which in the orientation of Figure 1 is directed vertically
downwards.
[0035] Consequently the first portion 9 of the strip 2 is biassed by the magnetic force
in a downward direction and rests in linear contact with the first wall 5.
[0036] A second magnetic element 15 is located externally of the enclosure 4 adjacent the
second wall 6 and is permanently magnetised in the same sense as the first magnetic
element 12 but to a lesser strength. Consequently a weaker attractive magnetic force
acts vertically upwardly on the strip so that the second portion to of the strip is
biassed vertically upwards and rests in linear contact with the second wall 6.
[0037] The deformed portion 11 is constrained to extend between the first and second portions
9 and 10 at a location which is longitudinally localised in registration with the
leading edge 14.
[0038] Longitudinal displacement of the element 12 to the left or right as viewed in Figure
1 will be accompanied by movement of the strip such that the deformed portion 11 remains
in registration with the leading edge 14. The longitudinal displacement of the deformed
portion 11 is therefore representative of the longitudinal displacement of the body
associated with the magnetic element 12. The position of a body may thereby be sensed
using the arrangement of Figure 1 by attaching such a magnetic element 12 to the body
in proximity with the sensor 1.
[0039] The position of the deformed portion 11 may be visually observed by providing the
enclosure 4 with a suitable window which preferably provides viewing through the second
wall 6. The chamber 3 may if required be filled with an opaque liquid such that a
visual demarcation is viewable in the window between that portion of the window contacted
by the strip 2 and that portion which is separated from the strip and therefore contacted
by opaque liquid. Various arrangements of illuminating such an enclosure and providing
measurement scales can also be incorporated into such a sensor.
[0040] The second magnetic element 15 may alternatively be replaced by an electrode to which
an electrostatic charge is applied by a suitable circuit in order to provide electrostatic
attraction between the strip 2 and the electrode in a direction which will bias the
strip into contact with the second wall 6.
[0041] The magnetic element 12 in a further alternative arrangement may comprise a magnetised
portion located at the leading edge 14 of sufficient strength to obviate the need
for the element to be magnetised along its entire length.
[0042] An alternative embodiment is shown in Figure 2 in which an alternative sensor 20
has a strip 2 which is resiliently deformed so as to remain biassed into contact with
the second wall 6. An end portion 21 of the second portion 10 of the strip is held
in a clamp 22 which grips the end portion such that it is deformed into adopting an
arcuate profile as viewed in longitudinal projection. The strip 2 will then tend to
remain biassed against the second wall 6 in opposition to the downward magnetic force
applied by the magnetic element 12. The second wall 6 is formed of transparent material
constituting a window through which the strip 2 can be observed and the chamber 3
is filled with opaque liquid.
[0043] The position of the deformed portion 11 may be sensed by other means instead of or
in addition to visual means. The first and second walls 5 and 6 may for example comprise
insulated conducting plates. By connecting the plates and the strip to a suitable
circuit the capacitances C₁ and C₂ between the first plate and the strip and between
the second plate and the strip respectively can be sensed and compared. An output
representative of C₁/(C₁+C₂) will for example provide a signal representative of the
fractional distance from one end at which the deformed portion is located.
[0044] Figure 3 illustrates a further alternative means of sensing the position of the deformed
portion 11. Conductive tracks 25 and 26 are arranged to extend in coaxial loops over
the first and second walls 5 and 6, the tracks being arranged such that magnetic flux
generated by current through one track will be coupled to the second track. By connecting
the tracks to a suitable circuit and energising one track with an alternating current
the inductive coupling between the tracks can be sensed and will be influenced by
the proximity of the electromagnetic strip (not shown in Figure 3 for clarity).
[0045] The strip 2 may itself be marked with a scale which can be read by a detector arranged
to read only that portion of the scale corresponding to the deformed portion 11. An
optical arrangement using light or non-visible radiation can be arranged such that
an illuminating beam is passed longitudinally through the chamber 3 so as to be incident
upon the deformed portion 11 as illustrated schematically in Figure 6 where a light
beam 60 is shown being transmitted and received from a transducer 61. In the arrangement
of Figure 6 the strip is constrained such that its S-bend configuration deforms the
strip to a maximum deflection of 90° relative to the longitudinal extent. There is
therefore a central portion of the deformed portion 11 which is substantially at 90°
to the light beam 60 thereby enabling light to be returned to the transducer. The
scale illuminated at the deformed portion 11 may be read by a suitable optical arrangement.
The effectiveness of such reading techniques will be enhanced by increasing the size
of that reflective part of the strip 2 which is maintained at substantially 90° to
the longitudinal extent of the strip and further improved by providing a positive
powered lens in proximity with the scale to be read. In such an arrangement it is
therefore advantageous to utilise a movable lens element 30 as illustrated in Figure
4 which is mounted on a carrier 31 arranged to move the lens element in unison with
the deformed portion 11.
[0046] The carrier 31 is mounted for sliding movement longitudinally of the chamber and
includes first and second guide rollers 32 and 33 which engage and guide the strip
2 in a manner which constrains the deformed portion 11 to extend linearly between
the guide rollers in a direction orthogonal to the walls 5 and 6 and hence orthogonally
to the beam 60.
[0047] The guide rollers 32 and 33 are arranged to contact upper and lower surfaces 34 and
35 respectively of the strip and are spaced apart in a direction orthogonal to the
plane of the strip so as to constrain the strip into its deformed state. The carrier
31 is formed of a low friction material which in the preferred embodiment is PTFE
so that sliding contact with the walls of the enclosure 4 incurs minimal resistive
drag.
[0048] Scale markings 36 applied to the strip 2 provide a binary code which for each illuminated
portion of the scale markings uniquely defines the position of the illuminated portion
with respect to the longitudinal extent of the scale. In the preferred embodiment
the scale is represented by a pseudo random binary sequence of black and white bars.
Alternative codes such as Gray code may be utilised.
[0049] Such a scale may alternatively be applied to the second wall 6 and may be read by
illuminating a portion of the scale using a beam which is deflected from the deformed
portion through 90°.
Such an arrangement requires the deformed portion to extend at substantially 45° to
the longitudinal extent of the enclosure. This can be accommodated by suitable spacing
of the end walls 7 and 8. The area available for reflection may be enhanced by using
a carrier 31 in which the guide rollers 32 and 33 are offset in longitudinal position
so as to dispose a substantially planar surface of the deformed portion 11 extending
at 45° relative to the longitudinal extent.
[0050] Such deflection through 90° may alternatively be utilised in an arrangement shown
in Figure 7 in which a fluorescent light guide 70 is placed along the second wall
6 and is illuminated by means of a longitudinally directed beam 60 which is reflected
through 90° so as to be incident upon the light guide in registration with the deformed
portion 11. The location along the light guide 70 at which fluorescence occurs is
thereby representative of the position of the deformed portion 11 and may be visually
or otherwise sensed to provide an indication of the displacement of the body.
[0051] A further alternative method of sensing the displacement of the deformed portion
utilises an arrangement of the type shown in Figure 6 in which a light beam 60 transmitted
so as to be reflected from the deformed portion is detected by a transducer 61 of
an optical ranging device utilised to sense the range of the deformed portion from
the transducer. A signal representative of the range is therefore representative of
the position of the deformed portion 11 and hence is representative of the displacement
of the body.
[0052] Sensors of the type disclosed above have numerous applications such as the sensing
of liquid level as illustrated schematically in Figure 5. A float 40 is arranged to
be vertically movable on the liquid surface and in proximity with the enclosure 4.
The strip 2 received within the enclosure has a deformed portion 11 which moves in
registration with a magnetic element 12 forming part of the float.
[0053] The position of the deformed portion 11 may be sensed by any of the techniques referred
to above to provide a liquid level measurement.
[0054] The use of an enclosure which seals the chamber 3 containing the strip 2 from external
environments renders the sensor particularly useful for applications in aircraft for
measurement of liquid levels and displacement of other moving bodies. Valve position
elements may also be remotely sensed using such sensors.
[0055] The chamber 3 may be filled with a suitable damping fluid to remove vibration effects
should this be necessary in a particular environment.
[0056] The magnetic means associated with the body may alternatively comprise an electromagnet.
[0057] Alternatively the strip itself may be permanently magnetised and the magnetic means
associated with the body may be a ferromagnetic element which is acted upon by the
strip.
[0058] In a further alternative, the strip may be placed in the field of a fixed permanent
magnet, a soft iron element being associated with the body so as to distort the field
in a manner which is depended on displacement.
1. A sensor (1,20) for sensing the displacement of a body (12) comprising a deformable
strip (2), a rigid enclosure (4) defining a chamber (3) within which the strip is
received and isolating the strip from contact with the body, constraining means (7,8,5,6;22)
operable to maintain the strip under longitudinal and transverse constraint such that
first and second end portions (9,10) of the strip extend substantially parallel to
a longitudinal direction in which the body is movable relative to the sensor and locally
deforming the strip in a transverse direction orthogonal to the first and second portions
such that a deformed portion (11) in the form of an S-bend integrally and unitarily
connects the first and second end portions, and first magnetic means (12) operable
between the strip and the body to apply a magnetic force non-uniformly to the strip
such that the location at which the strip is deformed to constitute the deformed portion
is longitudinally variable in registration with a reference portion (14) of the body
whereby the longitudinal displacement of the deformed portion is representative of
the displacement of the body.
2. A sensor as claimed in claim 1 wherein the strip is comprised of ferromagnetic material
and the body comprises a magnetic element (12) arranged in closer proximity to the
first end portion than to the second end portion thereby constituting the first magnetic
means.
3. A sensor as claimed in any preceding claim wherein the length of the chamber is less
than the undeformed length of the strip whereby end walls (7,8) of the enclosure constitute
longitudinally constraining element of the constraining means.
4. A sensor as claimed in any preceding claim wherein the enclosure comprises longitudinally
extending first and second walls (5,6) contacted by the respective first and second
end portions of the strip and wherein the first magnetic means is operable to bias
the first end portion into contact with the first wall.
5. A sensor as claimed in claim 4 further comprising biassing means (15) operable to
bias the second end portion into co-planar contact with the second wall.
6. A sensor as claimed in claim 5 wherein the biassing means comprises a second magnetic
means (15) operable to apply substantially uniformly to the strip a magnetic force
in opposition to and of smaller magnitude than the magnetic force applied by the first
magnetic means.
7. A sensor as claimed in claim 5 wherein the biassing means comprises electrostatic
means operable to apply an electrostatic force in opposition to and of lesser magnitude
than the magnetic force applied by the first magnetic means.
8. A sensor as claimed in claim 5 wherein the biassing means comprises a clamp (22) operable
on the second linear portion to apply a transverse deformation such that the second
linear portion exhibits an arcuate profile when viewed in longitudinal projection.
9. A sensor as claimed in any preceding claim wherein the magnetic element extends longitudinally
in a direction parallel to the direction of motion and wherein the magnetic element
is transversely magnetised along its length.
10. a sensor as claimed in any preceding claim wherein the enclosure comprises a window
through which the strip is visible.
11. A sensor as claimed in claim 10 wherein the chamber is filled with a liquid.
12. A sensor as claimed in any preceding claim further comprising sensing means (25,26;5,6)
operable to sense the position of the deformed portion and to produce an electrical
signal representative of the longitudinal displacement of the deformed portion.
13. A sensor as claimed in claim 12 wherein the sensing means comprises at least one electrode
(5,6) associated with the enclosure and wherein the sensing means is operable to produce
an electronic signal responsive to the capacitance between the electrode and the strip.
14. A sensor as claimed in claim 12 wherein the sensing means comprises two or more inductive
elements (25,26) associated with the enclosure and wherein the sensing means is operable
to produce an electronic signal responsive to inductive coupling between the elements.
15. A sensor as claimed in claim 12 wherein the sensing means comprises a transmitter
(61) of electromagnetic radiation operable to transmit longitudinally through the
chamber, a detector (61) operable to detect reflective radiation, ranging means operable
to generate an output signal representative of the range at which the radiation is
reflected and reflecting means associated with the deformed portion whereby the output
signal is representative of the displacement of the deformed portion.
16. A sensor as claimed in claim 15 wherein the constraining means is operable to induce
an S-bend at the deformed portion in which the strip is deformed through substantially
90° relative to its longitudinal extent.
17. A sensor as claimed in claim 12 wherein the sensing means comprises a longitudinally
extending fluorescent light guide (70), the sensor further comprising a light source
operable to illuminate the deformed portion and an optical detector operable to produce
a signal representative of the longitudinal location of the light guide at which the
fluorescence is detected.
18. A sensor as claimed in claim 17 wherein the constraining means is operable to deform
the strip such that the strip is deformed through substantially 45° at the S-bend
whereby a longitudinally directed light beam is transversely deflected through substantially
90° at the deformed portion.
19. A sensor as claimed in claim 12 wherein the sensing means comprises a scale (36) extending
longitudinally of the enclosure and optical reading means operable to read the portion
of scale in registration with the deformed portion.
20. A sensor as claimed in claim 19 wherein the scale comprises markings (36) applied
to the strip and wherein the deformed portion comprises an S-bend in which the strip
is bent through substantially 90° relative to its longitudinal extent.
21. A sensor as claimed in claim 19 wherein the scale comprises markings applied to a
wall of the enclosure and wherein the deformed portion comprises an S-bend in which
the strip is bent through substantially 45° relative to its longitudinal extent.
22. A sensor as claimed in claim 21 wherein the optical detector comprises one or more
lenses (30) or mirrors mounted on a sledge (31) coupled to the strip so as to be localised
to and movable with the deformed portion.